The present invention relates to a device for limiting the bend of a flexible pipe, particularly in cases where the pipe has a fitting of a diameter larger than that of the pipe, and more specifically a device comprising a stiffener intended to be fitted to a flexible pipe through which there flows a hot fluid, the temperature of which exceeds 80 .degree. C.
Such flexible pipes, the structure of which is well known, are used especially in the oil industry for raising or transporting hydrocarbons from a subsea well head to a platform or floating equipment such as a Floating Production and Storage Offloading boat known by the abbreviation FPSO. These flexible pipes are described in the document "Recommended Practice for Flexible Pipe--API Recommended Practice 17 B" published by the American Petroleum Institute.
The bend limiter according to the invention can also be used with other types of flexible pipelines, particularly in the case of subsea oil extraction installations, such as multi-pipe flexible lines known as umbilicals or electrical cables.
When the flexible pipe is working statically, as is the case when it is resting on the seabed, it is not very subject to bending. On the other hand, when it is working dynamically, as is the case when it is used as a riser, it is then subject to reverse deformations and especially bending which may lead to substantial bending of the said flexible pipe. Now, it is necessary for the Minimum Bend Radius (MBR) not to be smaller than a certain value (a reminder of this is given in the document API RP 17 B cited above). In order to limit the bend radius to an acceptable value, use is made of stiffeners which reduce the bending of the pipe at the critical points (stiffeners described in API RP 17 B by the name of "bend stiffener"). For example, a stiffener is fitted in the region where the flexible pipe is joined to the end fitting or fittings which are fitted to the flexible pipe at its two ends, an end fitting comprising, as described in API RP 17 B by the name of "end fitting", on the one hand, a flexible pipe termination member ("end termination"), and on the other hand, a member ("end connector") for connection to the corresponding pipelines to which the flexible pipe is connected, particularly a subsea well head and/or a support on the surface, such as a ship or a platform.
A stiffener may also be mounted on the running length of the flexible pipe, some distance from the end fittings, for example at the edge of an intermediate support ("pipe tray") in the case of a flexible pipe being used as a riser with a "lazy S" or "Steep S" configuration, as described in API RP 17 B, or alternatively, at the end of a rigid guide tube protecting the upper part of the flexible pipe, as depicted in FIGS. 11, 12 and 13 of WO 92/12376 or in EP-565,445 (Coflexip's build-in stiffener).
Such stiffeners are well known. They may consist of a single elastic body moulded in polyurethane and may alternatively comprise an internal reinforcing structure as described in WO 92/12376 or in GB-A-2,291,686. They may also comprise heat-dissipation means, as is recommended in FR-95 14114--not published--in the name of the Applicant, and GB-A-2,291,686. The heat-dissipation means are intended to dissipate some of the heat at the interface between the elastic body of the stiffener and the flexible pipe, the heat coming from the fluid flowing through the said flexible pipe. This is needed because it has been discovered that the heat damages the plastic (polyurethane) of the stiffener by hydrolysis in contact with the sea water, hydrolysis leading to a loss of mechanical properties of the stiffener, and especially to a loss of stiffness.
In all cases in which a stiffener is used, the stiffener is normally arranged on the flexible pipe before the end fittings are fitted and attached, radial clearance between the flexible pipe allowing the end fitting to be slid along the flexible pipe. Once the end fittings have been fitted to the flexible pipe, the stiffener is brought, by sliding, onto the corresponding end fitting and attached thereto by appropriate means which are at least partially described in the prior-art documents. Alternatively, if the stiffener is to be installed along the running length of the pipe, it is positioned at the desired point by temporary or permanent attachment means, such as a collar tightened around the pipe.
This being the case, it is the entire assembly consisting of the flexible pipe, the end fittings and the stiffener, which is transported to the place of use; this assembly is usually wound onto a reel or arranged in a basket for transport purposes. The winding onto a reel and subsequent transport pose serious problems which hitherto have not been adequately solved. A problem is that independently of the weight of the flexible pipe and of the end fittings, it is important to note that each stiffener may have a length which may be of the order of four meters long and combined with the end fitting constitute a localized load that can be as much as about three tons. This results in problems of bulk, of balance and of handling which seriously affect all of the implementation operations from the end of manufacture of the pipe and its loading onto a ship, as far as installing it at the off-shore site. In particular, getting the stiffener to pass through the various pipe-handling members, such as tracked tensioners, or curved supports or the grooves of pulleys, requires tricky and lengthy manipulation.
In practice, the size of the stiffeners is currently limited by these problems of handling which are caused by the fact that they are fitted to the flexible pipe. Now, the continuing increase in the diameter and the weight is of the flexible pipes and in the depth of water means that there is now a need for even heavier stiffeners, and ones whose length would exceed five meters, something which is practically unachievable on account of the handling difficulties.
In order to overcome these difficulties, it would be natural to envisage fitting the stiffeners in situ at the time the flexible pipe is installed on board the laying ship. An installation for overmoulding around the pipe is obviously excluded, because of the length of time needed for the operation and because of the surrounding conditions. From another source, stiffeners are known which are made in two halves which can be fitted at the last minute and joined together around the flexible pipe by means such as bolt-on collars, as described in FR-2,446,981, but experience has shown that a stiffener of this kind is unable to exhibit the required mechanical characteristics, irrespective of the direction of bending of the pipe.